A variety of reaction modes are available to alcohols.

Reactions of Alcohols with Base: Preparation of Alkoxides 9-1 Strong bases are needed to deprotonate alcohols completely. Base strength must be stronger than that of the alkoxide.

Alkali metals also deprotonate alcohols, but by reduction of H +. Vigorous: Less Vigorous: Relative reactivities:

Uses for alkoxides: Hindered alkoxides E 2 reactions with haloalkanes to form alkenes. Less hindered alkoxides S N2 reactions with haloalkanes to form ethers.

Reactions of Alcohols with Strong Acids: Alkyloxonium Ions in Substitution and Elimination Reactions of Alcohols 9-2 Water has a high pK a (15.7) which means that its conjugate base, OH - is an exceedingly poor leaving group. The –OH group of an alcohol must be converted into a better leaving group for alcohols to participate in substitution or elimination reactions.

Reactions of Alcohols with Strong Acids: Alkyloxonium Ions in Substitution and Elimination Reactions of Alcohols 9-2 Haloalkanes from primary alcohols and HX: Water can be a leaving group. Protonation of the hydroxy substituent of an alcohol to form an alkyloxonium ion converts the –OH from the poor leaving group, OH -, to the good leaving group, H 2 O.

Primary bromoalkanes and iodoalkanes can be prepared by the reaction with HBr and HI. Chloroalkanes cannot be prepared by this method because Cl - is too poor a nucleophile.

Secondary and tertiary alcohols undergo carbocation reactions with acids: S N 1 and E1. Primary alkyloxonium ions undergo only S N 2 reactions with acid. Their carbocation transition state energies are too high to allow S N 1 and E1 reactions under ordinary laboratory conditions. Secondary and tertiary alkyloxonium ions lose water when treated with acid to form a carbocation.

When good nucleophiles are present, the S N 1 mechanism predominates. Here the tertiary carbocation is generated at a relatively low temperature, which prevents the competing E 1 reaction. At higher temperatures, or in the absence of good nucleophiles, elimination becomes dominant.

Secondary alcohols show complex behavior when treated with HX, following S N 2, S N 1, and E1 pathways. Relatively hindered (compared to primary alcohols): Retarded S N 2 reactivity Slow to form carbocations (compared to tertiary alcohols): Retarded S N 1 reactivity.

E1 reactions of alcohols (dehydrations) result in the formation of alkenes. Non- nucleophilic acids, such as H 3 PO 4 or H 2 SO 4, are used in this case, rather than the nucleophilic acids, HBr and HI. Dehydrations of tertiary alcohols often occur just above room temperature.

Summary